The present invention relates to hydrostatic or positive displacement pumps and, more particularly, to external gear pumps.
Gear pumps have therein a pair of rotating gears with gear teeth that come into, and then leave, a meshing with those of the other. They thereby continually trap fluid portions at one location and displace those fluid portions to another location so, as a result, to effect a pumping of that fluid. The cross section side view schematic diagram of FIG. 1A shows an external gear pump, 1. Such a pump typically has two identical spur gears, 2 and 3, each with gear teeth arrayed about the outer periphery thereof and each mounted on, or integrally supported by, a corresponding one of a pair of gear shafts, 4 and 5, with these gears contained in a pump housing, 6. The teeth of these gears mesh with one another at a mesh location, 7, through having one or more gear teeth of one coming into, and leaving, mesh with one or more gear teeth of the other because of those gears being rotated in selected rotation directions by a selectively operated motor (not shown). When operated gear 2 is rotating clockwise, and so resulting in gear 3 rotating counterclockwise, pump 1 draws the fluid to be pumped through an inlet opening, 8, in housing 6, as indicated by the broad, flat arrow shown there, to be transported by rotating gears 2 and 3 along the interior of the walls of housing 6 at the outer periphery of those gears to an outlet opening, 9, in housing 6 at which the fluid exits the pump as indicated by the broad, flat arrow shown there.
FIG. 1B is a perspective schematic diagram providing more detail of the internal mechanisms of pump 1 through presenting same outside of housing 6. The figure shows a portion of a motor drive shaft, 10, extending from the unseen operating motor through the side of housing 6 to connect to gear shaft 4 (which alternatively may merely be extended to form drive shaft 10), as the basis for that motor to force rotation of gear 2 mounted on that shaft in the rotation direction selected therefor. Here, gear 2 is shown rotated in the clockwise direction to be consistent with the fluid flow direction along the primary paths of the fluid being pumped through its being transported from the intake side of pump 1 to the discharge side thereof which are shown by broad, flat arrows in FIG. 1B. This rotation of gear 2 in turn forces gear 3 on gear shaft 5, through the meshing of the two gears at mesh location 7, to also rotate but in the opposite rotation direction, or counterclockwise. Gear shafts 4 and 5, each extend at both of the opposite outer ends thereof into one of a corresponding pair of bearings, 11.
Each of bearings 11 comprises a ring-like structure that has a flat, but partially recessed, bearing surface, 11′, facing the one of gears 2 and 3 it is supporting. Thus, such bearing surfaces extend substantially perpendicular to the direction of extent of the corresponding one of gear shafts 4 and 5 passing therethrough in their extending along the corresponding shaft axis of symmetry. Further, each of bearings 11 has a circular cross section bushing, 11″, in the center of its ring-like structure which extends perpendicular to the bearing surface it intersects in being aligned with the symmetry axis of the gear shaft positioned therein.
Since the two ring-like structures of bearings 11 on the same side of each of the gears are closely adjacent to one another, they can be, and usually are, structurally or integrally joined together in a structure resembling a “figure 8” when viewed from a direction parallel to the axes of symmetry of the gear shafts when positioned therein. The pairs of bearings 11 at opposite ends of the gear shafts for each of the gears or, alternatively, the pair of “figure 8” structures 11 at the opposite ends of the gear shafts of both gears, are held in housing 6 so that this housing surrounds the gears and those bearings. As indicated above, gear shaft 4 is connectable to, or extendable as, motor drive shaft 10 in one or the other extending through the housing wall. When assembled in this housing, there is for the most part very little clearance between the flat parts of the bearing surfaces and the corresponding bearing surface side of the gear across therefrom to provide one basis for keeping the fluid being pumped from escaping out the sides of the gears.
At the intake side of pump 1, inlet opening 8 in the wall of pump housing 6 forms an inlet port at which fluid to be pumped is drawn to enter by gears 2 and 3 coming out of mesh at a location relatively near to this port. In coming out of mesh, an expanding inter-tooth volume forms between adjacent teeth on each gear as the formerly meshed tooth of the other gear exits those spaces. These inter-tooth volumes in the spaces between adjacent teeth on the gear coming out of mesh are filled by fluid from the input port and, as indicated above, forced to move with each gear between its teeth along the closely adjacent interior surface of the outer wall of the housing to outlet opening 9 at the discharge side of the pump. The very small clearances between the tips of the teeth on the gears and the corresponding housing wall interior surface, the speed of movement of the gear teeth tips along that surface, and the close proximity of the flat bearing surfaces to the sides of the gears, as described above, keep the fluid in the inter-tooth volumes trapped to prevent same from leaking backward towards the input port.
At the discharge side of the pump, outlet opening 9 in the wall of housing 6 forms an outlet port at which fluid is being forced to exit by gears 2 and 3 going into mesh at a location relatively near to this port to form shrinking inter-tooth volumes between those adjacent teeth on each gear resulting from corresponding teeth of the other gear entering those spaces. As a positive displacement pump, the fluid discharge pressure is predominantly determined by the downstream conduit passageway cross sectional areas. The meshing of the teeth of gears 2 and 3, at meshing location 7 which is more or less along an axis there joining the axes of symmetry of gear shafts 4 and 5, and the presence of closely adjacent flat bearing surface portions there, has the effect of isolating the fluid at the output port from that at the input port.
Cavitation can occur in external gear pumps on the intake side of the pump in a region, 12, in which the teeth of gears 2 and 3 separate in coming out of mesh with one another. In this region, as indicated above, the expanding inter-tooth volume between adjacent teeth on each gear, where a tooth of the other gear had just been and is exiting, must be filled by the fluid to be pumped that is coming in from inlet opening 8 under whatever is the inlet port fluid pressure. As the rotational speed of the gears increases to reach some threshold value the rate of the expanding inter-tooth volumes can exceed the rate such volumes can be filled by this incoming fluid at inlet port 8 under the inlet port fluid pressure. In these circumstances, the local fluid pressure decreases below the vapor pressures of dissolved gases in the fluid, or the vapor pressure of the pumped fluid itself, so as to rupture the continuity of the fluid at some particle or solid surface nucleation site and thereby form a cavity or bubble. Such gases, or the vapors of the fluid, or both, evaporate into that cavity from the surrounding fluid medium.
As the inter-tooth volumes subsequently become more filled, the rising local fluid pressure forces such cavities or bubbles toward collapse causing the pressure and the temperature of the vapors therein to increase. This continues until the volume of those cavities or bubbles become a very small fraction of their original sizes to finally reach a point of total collapse, and so to result in an acoustic shock wave occurring in a very small volume that dissipates the vapors into the surrounding fluid medium. Such collapses occurring on or near surfaces of the gear teeth can erode them to thereby leave pits at those surfaces which, in occurring repeatedly, can be very destructive of the gear teeth surfaces.
Because of occurrences of such unwanted cavitation, bearing surfaces 11′ have often been recessed inward into the bearing to have those bearings be provided with channels, 11′″ and 11″″ (not seen in FIG. 1B), therein that begin adjacent to location 7 where gears 2 and 3 mesh and, from there, extend along generally opposite directions. These opposite directions are both substantially perpendicular to an axis intersecting the axes of symmetry of shafts 4 and 5 positioned in cross section bushings 11″, and the channels extend along these directions to corresponding ones of outer edge portions of bearing ring-like structures, or the “figure 8” structures, 11. At these outer edges, such channels may extend over a circular arc that is an eighth or more of the circular outer edge. Thus, there are two such channels, input channel 11′″ and output channel 11″″ (not seen in FIG. 1B), each directed from a corresponding beginning location near location 7 and extending in opposite directions to each terminate at an outer edge of structure 11 near, respectively, a corresponding one of inlet 8 and outlet 9. Output channel 11″″ (not seen in FIG. 1B) accommodates the pumped fluid being squeezed out between the gear teeth coming into mesh near outlet 9, and input channel 11′″ accommodates the pump incoming fluid rushing in between the gear teeth coming out of mesh near inlet 8.
Even with such accommodations, however, the rate at which the returning fluid fills the expanding inter-tooth volume depends on the fluid pressure at the inlet port. Hence, beyond some rotation rate, this fluid inter-tooth volume filling rate will be insufficient to keep up with the expanding inter-tooth volume rate so as to still result in cavitation occurring. Thus, there is a desire for a gear pump with an arrangement for reducing further, or eliminating, cavitation occurrences during operation thereof.